CHAPTER XXIII 



CARBOHYDRATES 



The carbohydrates, so called because they are built up of 

 carbon and water, are a varied group including such diverse 

 substances as sugar, starch, cellulose, agar, and vegetable gums. 

 The mere statement that the sugars are the chief source of energy 

 liberated through respiration in plants and animals, that starch 

 is an important form of stored energy in the plant and source of 

 energy in animal food, and that cellulose constitutes the frame- 

 work of higher plants is sufficient to indicate the great role played 

 by the carbohydrates in organisms. Food is translocated 

 (shipped) in the plant primarily in the form of sugars, and the 

 osmotic value of cells is maintained chiefly by sugars. Among 

 the constituents of protoplasm, sugars are exceeded in impor- 

 tance only by water and the proteins. 



The carbohydrates are classified as follows: 



I. Sugars 



A. Monosaccharides, or simple sugars 



(Formaldehyde, CH2O, is the first homologue of the series, 

 but it is not a sugar) 



1. Pentoses, C5H12O0 — arabinose. 



2. Hexoses, C6H12O6 — glucose or dextrose (grape sugar), 

 fructose or levnlose (fruit sugar), galactose, mannose. 



B. Disaccharides, or compound sugars, C12H22O11 — sucrose or 

 saccharose (cane sugar), maltose, cellobiose, lactose (milk 

 sugar). 



C. Trisaccharides, C18H32O16 — raffinose. 



II. Nonsugars 



A. Starches (C6Hio06)x — starch, dextrin, glycogen. 



B. Gums (CgHioOo)^ — vegetable gums, mucilages, pectins, pento- 

 sans (CsHloOs)!/. 



C. Cellulose (C6Hio05)z — normal (a) cellulose, lignocellulose. 



The examples given in any one group in the above table have 

 the same basic formula for that group, but they may differ 

 greatly in other respects; thus, glucose and fructose both have 



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